Polysilicon surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographically patterning of alternate layers of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material (e.g. silicon dioxide). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer using a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the silicon dioxide layers using a selective etchant such as hydrofluoric acid (HF) which does not attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane, and up to three or more additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams to complex systems such as an electrostatic motor connected to a plurality of gears. Typical in-plane lateral dimensions of the functional elements can range from one micron to several hundred microns, while the layer thicknesses are typically about 1-2 microns. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
The present invention relates to a microelectromechanical (MEM) timer formed from silicon micromachining technology using MEM electrostatic motors of the type disclosed by Garcia et al in U.S. Pat. No. 5,631,514 which is incorporated herein by reference. In the present invention, a first MEM electrostatic motor is used to intermittently wind a mainspring of the MEM timer. The MEM timer drives a set of meshed timing gears that are encoded so that timing information that can be optically read out. The present invention can also include a second electrostatic motor for starting and stopping the MEM timer.
An advantage of the present invention is that a compact and rugged MEM timer can be formed which, once activated, provides accurate timing information through an optical readout and retains the timing information even though electrical power to the device may be temporarily interrupted.
Another advantage of the present invention is that the MEM timer can provide millisecond timing resolution, and a running time of up to an hour or longer depending upon the number of timing gears provided in a mechanically-driven gear train and how often the mainspring is rewound.
Yet another advantage of the present invention is that the MEM timer provides an optical readout of timing information that can be accessed optically by a plurality of light beams, including light-emitting-diode (LED) or laser beams, transmitted through free space or optical fibers.
Still another advantage of the present invention is that preferred embodiments of the MEM timer can be fabricated without the need for piece part assembly.
These and other advantages of the method of the present invention will become evident to those skilled in the art.